1. Field of the Invention
The present invention relates to mirrors, particularly to a vehicle rearview mirror which blocks excessive light levels, e.g., caused by headlight glare.
2. Description of Prior Art
During night driving, when a first car is followed by another car which has its bright lights on, these bright lights are reflected from the rearview mirror into the eyes of the car's driver. This can seriously impair the driver's forward vision because the light scatters within the driver's eyes, causing a "veil" or "curtain" of light over the scene. The driver therefore has reduced ability to detect objects which are dimly lit or have little contrast with the background. This situation is known as visibility glare. The driver is normally unable to detect this loss of visibility since it produces no physical sensation. At higher reflected light levels, this glare becomes uncomfortable, resulting in an unpleasant physical sensation in the driver's eyes.
To alleviate this problem, manually actuated rearview mirrors have been developed which include "day" and "night" settings. These day-night mirrors are installed almost in all modern cars and include front and rear mirrors joined at an angle to form a prism and a mechanism for changing the angular orientation of the prism. The front mirror is half-silvered. In the day setting, the rear mirror is angularly set to the working position to provide approximately 80% reflectance. In the night setting, the front mirror is angularly set to the working position to provide only approximately 4% reflectance.
Additionally, automatic day-night rearview mirrors have been developed which automatically switch the mirror prism between full and partial reflectance states in response to sensed light levels. One such mirror is illustrated in U.S. Pat. No. 4,443,057, issued Apr. 17, 1984, to Bauer et al. The Bauer mirror includes a forward light sensor for measuring light in the forward direction, a rear light sensor for measuring light in the rear direction, and a control circuit responsive to the forward and rear light sensors to control the mirror prism.
These automatic mirrors all suffer a common drawback. Specifically, the light sensors used in these mirrors have a response to the electromagnetic spectrum which is substantially different from the spectral response of the human eye. Accordingly, the sensitivities of the light sensors to visible and invisible wavelengths are different from the sensitivities of the human eye. Most notably, light sensors are extremely sensitive to infrared and longer wavelengths which cannot be seen by the human eye. Infrared wavelengths are prevalent in artificial lighting, particularly in tungsten filament bulbs. The light sensors detect relatively high light intensities when viewing headlights, taillights, streetlights, or any other source of infrared wavelengths. Consequently, the reflective element of a mirror incorporating such sensors is driven to an inappropriate reflective state. The mirror therefore is sometimes actuated when not necessary to meet the sensitivity of the human eye. As a result, either inadequate image information is presented to the driver and/or excessive glare from the rearview mirror is directed to the driver's eye.
An attempt has been made to obviate the disadvantages of the above-mentioned light sensors by utilizing filtered light sensors of the type described in U.S. Pat. No. 4,799,768, issued Jan. 4, 1989, to E. Gahan. The Gahan light sensor includes a detector responsive to electromagnetic wavelengths and a filter for filtering the wavelengths received by the detector, so that the spectral response of the light sensor approximates the spectral response of the human eye. Nevertheless, this rearview mirror switches the entire surface of the mirror into the "night" state. Furthermore, this mirror is based on the use of light sensors, and such sensors essentially attenuate the incident light to a certain level, or otherwise reflect the incident light in a certain direction. This causes distortion of a reflected image.
Another disadvantage, common to all two-position prism mirrors, is that such a prism is strictly a dual reflectance device which allows no option for a continuously variable reflectance or for intermediate reflectance states. Dual reflectance mirrors are highly inadequate because they distort and shift images.
A glare-free reflection mirror is shown in U.S. Pat. No. 4,721,364, issued Jan. 26, 1988 to H. Itoh et al. This mirror has an electro-optical element, the transparency of which is changed by applying an electrical field to this element. The mirror is divided into a dazzle-free zone in the lower part of the mirror's surface and a non-dazzle-free portion at the rest of the mirror's surface. Such rough regional localization of the
mirror's surface reduces the field of vision at nighttime. Furthermore, the Itoh mirror possesses the same disadvantages as the other sensor-type mirrors described above.
Recently, rearview mirrors using liquid crystal devices having light absorption properties have been designed. One such nonglare mirror is described in U.S. Pat. No. 4,671,617, issued Jun. 9, 1987 to K. Hara. Mirrors of this type incorporate a liquid crystal device. In this device, the orientations of the liquid crystal molecules are changed to absorb light when the amount of the light incident on the mirror exceeds a certain limit.
There are many other dazzle-free mirrors based on the use of liquid crystals which are described in various publications. Each such mirror is aimed at the elimination of certain drawbacks of the existing liquid-crystal mirrors, such as: the prevention of a chemical reaction in the dichromatic dye of the liquid crystal (U.S. Pat. No. 4,848,878 issued Jul. 18, 1989 to E. Lee et al.); the provision of a control device to automatically initialize the antidazzle mirror to a selected mode of a predetermined antidazzle or dazzle state, when power is applied from a battery (U.S. Pat. No. 4,786,145 issued Nov. 22, 1988 to H. Demura, et al.); the elimination of interference fringes which often occur under monochromatic light sources, such as sodium or mercury lamps (found over highways), or the halogen lamps of automobiles (U.S. Pat. No. 4,729,638 issued Mar. 8, 1988 to Y. Shirai), and so forth.
A common disadvantage of all existing liquid-crystal dazzle-free mirrors is that they cannot provide efficient attenuation of the dazzling light. This is because the light attenuation effect is distributed over the entire surface of the mirror, causing the entire mirror to become dim even through a bright light shines in only a small area of the mirror.
An attempt has been made to solve the above problem by providing a liquid-crystal matrix-type reflection mirror with a localized dazzle light attenuation zone. The construction of this mirror is described in U.S. Pat. No. 5,168,378, issued Dec. 1, 1992 to Michael Black, et al.
The mirror has a multilayered structure and consists of a broadband reflective base mirror having maximum reflectivity in the range corresponding to the spectral range of halogen lamps of automobile headlights. Deposited on the base mirror are a matrix transparent electrode and a photoconductive layer placed on the matrix transparent electrode. The photoconductive layer and the matrix transparent electrode have a matrix-type structure formed as a pixel array. Each pixel of the matrix transparent electrode is a projection of an overlaid pixel of the photoconductive layer.
The multilayered structure further includes the following subsequent layers: a rear polarizer which has a predetermined axis of polarization and is placed on the photoconductive layer, a common transparent electrode, a liquid crystal sandwiched and sealed between the rear polarizer and the common transparent electrode, a front polarizer, and an antireflection coating.
The mirror has a control circuit connected between the common transparent electrode and the matrix transparent electrode. The control circuit applies a control voltage to the liquid crystal in accordance with the incident light. The pixel enables just the dazzle light zone to be attenuated.
Although the mirror described in this patent will solve the problems described above for concentrated light beams, such as a laser beam, it will not work with optimal efficiency in the case of a vehicle rearview mirror. This is because a diverging beam of automobile's headlights, which reaches the surface of the rearview mirror of a particular vehicle, is much wider than the surface of the mirror itself. In other words, the entire rearview mirror in front of the driver will be illuminated by the beams of the car behind it.
Moreover, the driver sees the reflection of the headlights, and the position of this reflection in the mirror depends on the position of the driver's eyes. In other words, when the driver's head moves, the driver sees the reflection of the headlight in a different place of the mirror, while the entire mirror remains illuminated with a wide beam of the headlights, the width of which in the plane of the mirror is much wider than the width of the mirror. The distance from the driver's eyes to the plane of the mirror is much shorter than the distance from the light source to the plane of the mirror. Therefore a change in the position of the driver's eyes will change the position of the reflected image of the light source seen by the driver to a greater extent than would result from a change in the position of the light source. Thus, the principle of attenuation of one bright point or small zone on the mirror does not work optimally for non-concentrated light beams, such as those emitted by automobile headlights.
Furthermore, since the rearview mirror is located close to the driver's eyes, due to the binocularity of human vision, the driver sees the reflection of the dazzling spot as two images, which are converted in the brain into a single image.
More specifically, when a driver looks with both eyes at a small object located remotely, e.g., at a distance of several meters, the driver sees this object with both eyes as a single image. When this object is moved closer to the viewer's eyes, at a certain distance the viewer sees two images (one for each eye). Therefore, the driver will subconsciously see the dazzling spot. Strictly speaking, the driver sees two dazzling spot images, one with each eye. Thus, even if only a local area of the mirror, corresponding to the reflection of the dazzling point is attenuated, the reflected image of this dazzling spot will not be attenuated completely.
Another disadvantage of this mirror is that it has only two stable conditions, i.e., an attenuated state and a non-attenuated state. In other words, the control voltage of the active matrix is determined by a threshold device and is switched over discretely between the attenuated and the non-attenuated state.